The invention relates to a membrane useful in facilitated transport processes.
The unit operation of facilitated transport, generally, involves a membrane in which a chemical species, termed a carrier, chemically complexes with another species and facilitates its transport across the membrane. Because the transport process is governed by the reaction rate for complex formation, as well as by the diffusion rate of the complex, chemical separations often can be effected with very high selectivity.
As reported in Noble, Richard, et al., "Facilitated Transport Membrane Systems", Chemical Engineering Progress, p. 58 (March, 1989), many chemical separation processes are based on a difference in a physical property: for example, distillation, crystallization, centrifugation, and gas absorption. These processes work well at separating and concentrating different solutes when there is a large variation in the physical property of interest and high selectivities are not required.
To overcome the limitations of these processes, a chemical agent is sometimes used. This chemical agent can selectively interact with one of the solutes in the feed mixture. The solute is then captured and concentrated. The solute-chemical agent complex is then reversed by a change in operating conditions to recover both the solute and the chemical agent.
An example of this approach is the use of amines in aqueous solution to remove acid gases (CO.sub.2, for example) from natural gas streams. The amine can complex with CO.sub.2 and causes a larger effective solubility of CO.sub.2 in the aqueous stream than would be obtained without the amine present. The aqueous solution is then heated in a separate vessel to remove the CO.sub.2 and the amine solution can be recycled.
An immiscible liquid can serve as a membrane between two liquid or gas phases. Different solutes will have different solubilities and diffusion coefficients in this liquid, and the product of these two terms is a measure of the permeability. A liquid membrane can yield selective permeabilities and, therefore, a separation. Because the diffusion coefficients in liquids are typically orders of magnitude higher than in polymers, a larger flux can be obtained.
Basic configurations of facilitated transport membranes include: (1) emulsion liquid membranes in which a bulk, continuous liquid phase serves as the membrane separating a source from emulsified liquid globules which are the receiving phase; (2) immobilized liquid membranes in which the liquid membrane phase can be supported in a porous film or dissolved in a polymer film or between permeable films; and (3) ion exchange membrane supports which are swollen by the liquid, herein referred to as the solvent. Major disadvantages of the emulsion liquid membrane systems include difficulties associated with membrane stability and process de-emulsification necessary to recover the transported species from the internal phase. Immobilized liquid membranes supported on a porous structure or dissolved in a polymer substantially eliminate these difficulties but these membranes suffer loss of the solvent or of the carrier and subsequent deactivation. Also, for liquid phase separations, it is necessary to choose both a solvent and a carrier that are immiscible or insoluble in either the feed or receiving phases. The use of ion exchange membranes as supports for immobilized liquid membranes overcomes some of the disadvantages of immobilized liquid membranes. Ion exchange membranes (IEM's) can be swollen with a relatively large quantity of liquid, usually water, which opens up the IEM polymer structure and increases the mobility of the carrier complex. Although the IEM's have great capacity for retention of hydrophilic solvents such as water, it is desirable to keep the IEM film saturated by humidifying the feed stock. More importantly, IEM's will retain the carrier by strong electrostatic forces and higher carrier loadings can be achieved and maintained. In a conventional immobilized liquid membrane, the carrier loading is determined by its solubility in the liquid phase. In an IEM, the carrier loading is determined by the ion exchange density.
Effective use of membranes for facilitated transport operations, as for other membrane separation processes, requires incorporation of a relatively large area of membrane into a compact module. The module should provide for an upstream compartment with feed and exit ports wherein the fluid mixture to be separated is introduced and can be swept across the face of the membrane such that the fluid composition at the surface of the membrane is constantly renewed. The module should also provide for a downstream compartment on the other side of the membrane, with feed and exit ports, whereby the fluid species that are preferentially transported or separated can be swept away as they arrive at the membrane surface.
The use of polymeric membranes in modules for the separation of fluid mixtures by differential permeation is well known. Devices have been made using a shell and tube construction with the polymeric membrane in hollow fiber or tubular form Also, devices have been made from spirally wound sheets separated by meshes to provide for upstream and downstream compartments. The spirally wound sheet and mesh array is normally potted or glued at each end to seal off the module so as to create upstream and downstream compartments of the module, each side fitted with feed and exit ports. The result is that the only route of a chemical species from the upstream side to the downstream side is through the membrane.
Preparing a module from a polymer film which is highly swollen with solvent is difficult. To create a leak-free module, the edges of the spiral wound polymer film must be glued or potted in the presence of large amounts of solvent in the polymer film. In the case of a relatively inert fluoropolymer membrane, special heat treatment above the boiling point of the solvent, i.e., water, and special surface treatments may be required to get a good, leak-free bond. In the presence of a large quantity of solvent used to swell the membrane, these bonding techniques become difficult.
Preparing a module from a unswollen polymer membrane and then swelling the membrane with solvent in situ also has drawbacks. A typical membrane may swell 20-50% by volume, and special treatments have been used to achieve as much as a 200% increase in volume. This creates large dimensional changes. When the membrane is rigidly held in a module, as the swelling step is carried out the membrane changes dimensionally causing it to warp and buckle. The swelling may be great enough to produce ruptures and tears in the membrane. If the membrane does not rupture, the warps and folds produced from swelling may produce areas of reduced or blocked flow, thereby greatly reducing the membrane's ability to transport material.
Instead of glueing or potting the edges of the spirally wrapped array, an interleaving of gasketing ribbon at each edge, coupled with mechanical clamping, has been considered. However, highly swollen IEM's are extremely fragile and could be broken by the clamping forces required.
U.S. Pat. No. 4,737,166 discloses a composite immobilized liquid membrane suitable for acid gas scrubbing. The membrane is a continuous, solvent swollen polymer film on a microporous polymeric support, the solvent being selected from a class of highly polar solvents. No carrier is involved nor are ion exchange resins or fluoropolymer films discussed.
Way and Noble have reported on "Hydrogen Sulfide Facilitated Transport in Perfluorosulfonic Acid Membranes", in Chapter 9 of Liquid Membranes, Theory and Applications, proceedings of the 8th Rocky Mountain Regional Meeting of the American Chemical Society, June 8-12, 1986. The IEM's involved were evaluated as small, single layer sheets which could be mounted in the test apparatus after swelling. The IEM's were about 7 mils thick before swelling With solvent and were in the perfluorosulfonic acid form. Water was the solvent and ethylene diamine was the carrier. These relatively thick perfluorosulfonic acid polymer (PFSAP) membranes were deficient in that transport rates were relatively low. Module fabrication by this method is difficult. These relatively thick PFSA membranes are difficult to bond when swollen with water, and cannot be swollen significantly after module fabrication without distortion and damage to the operability of the module. Mechanical clamping of the module to effect the necessary seal is not viable because of the fragility of the swollen structure.
J. D. Way, R. D. Noble, D. L. Reed, G. M. Ginley and L. A. Jarr have reported on "Facilitated Transport of CO.sub.2 in Ion Exchange Membranes" in the AICHE Journal, March 1987, Vol. 33, No. 3, pp. 480-487. Water was the solvent and ethylene diamine was the carrier. The perfluorosulfonic acid polymer film used was 7 mils thick.
R. D. Noble, J. J. Pellagrino, E. Grosgeat, D. Sperry and J. D. Way have reported continued studies of "CO.sub.2 Separation Using Facilitated Transport Ion Exchange Membranes" in Separation Science and Technology, 23 (12 & 13), pp. 1595-1609, 1988. In that study, higher rates of transport were achieved with a 1 mil thick perfluorosulfonic acid polymer film compared to a 7 mil thick perfluorosulfonic acid polymer film. In both of these studies, the PFSAP film was evaluated as a small, unwound single layer mounted in the test apparatus after swelling, and module construction was not considered. Transport rates with the 7 mil thick structure were relatively slow and swelling was too great to permit module construction prior to swelling. A one mil thick PFSAP membrane is very weak mechanically when swollen with water, and will not survive module assembly or sustained operation.
J. Pellegrino, R. Nassimbene and R. D. Noble have reported on additional studies in "Facilitated Transport of CO.sub.2 Through Highly Swollen Ion Exchange Membranes: the effect of hot glycerine treatment" in Gas Separation and Purification, Vol. 2, pp. 120-130, September 1988. In that paper, the authors describe a facilitated transport evaluation of a PFSAP film in a small, single layer unwound configuration involving water as the solvent and ethylene diamine as the carrier. The PFSAP membrane is swollen to a greater level than normally hydrated PFSAP membranes by swelling the membrane in glycerine at high temperatures prior to final immersion in water to replace the glycerine. This greater level of swelling opens up the structure so that the flux obtained is four to six times higher than previously observed for normal hydrated PFSAP membranes. A high degree of facilitation for CO.sub.2 and H.sub.2 S is maintained as well as a high degree of selectivity compared to non-carrier reactive gases. The 7 mil PFSAP films swelled both in thickness and in linear dimension, too much to permit swelling after module fabrication and maintain a functional module. Swollen PFSAP films are too weak to withstand clamping pressures necessary for a mechanical seal. A one mil highly swollen PFSAP film is more fragile, and increases greatly in linear dimension upon swelling, making it unsuitable for reliable module fabrication and operation.
C. A. Koval and T. Spontarelli, in Polymer Material Science Eng. 1988, V-59, pp. 132-138, reported on a facilitated transport technique wherein the Na+ ions in a perfluorosulfonate IEM are replaced by Ag+ ions in water swollen membranes. In that study, the flux of olefins such as 1-hexene and 1,5 hexadiene was enhanced substantially. The authors demonstrated that the reversible complex formation of olefins with aqueous Ag+ allows for the facilitated transport separation of olefins that bind strongly to Ag+ from sterically hindered and saturated hydrocarbons. However, water swollen, thin perfluoro ion exchange membranes are very weak mechanically and would be difficult to incorporate in a module and operate.
"Perfluorinated Ion Exchange Membranes" by Grot et al, presented to the Electrochemical Society, May 7-11 (1972), points out that the amount of water a membrane will absorb depends on the temperature of the water. A membrane pretreated at high temperature will continue to absorb that same amount of water at room temperature unless the effect is destroyed by drying at elevated temperatures. For example, boiling a 1200 equivalent weight sulfonic acid polymer in water causes 25% water absorption. While unreinforced membranes exhibit 14-17% linear growth, membranes in which a fabric is imbedded show linear growth as low as 3%. However, the effective cross-section is reduced to about 50%, swelling in the thickness direction is limited by the three-dimensional reinforcement, and carrier concentration and mobility would be relatively low in a facilitated transport operation. The only application mentioned in this paper was electrochemical cells.
U.S. Pat. No. 4,194,041 describes a flexible, layered composite article, which permits transfer therethrough of water vapor but not liquids, comprising a flexible layer of hydrophobic material such as EPTFE attached to a continuous hydrophilic layer such as a perfluoro ion exchange membrane. The hydrophobic layer provides poor access of hydrophilic liquids to and from the membrane surface. Drying of fluid streams was not discussed nor was the use of the composite for facilitated transport disclosed.
U.S. Pat. No. 4,604,170 discloses a composite membrane structure used for electrolysis which comprises a porous layer of fluorine containing polymer having its interior and anode-side surface being hydrophilic and having a thin ion exchange resin layer supported on the cathode-side thereof. Application of such a structure as a means of separating fluids in the absence of an electric field by differential permeation or by facilitated transport is not disclosed.
U.S. Pat. Nos. 4,666,468 and 4,741,744 describe a process for separating gases by differential permeation through a hydrated perfluorosulfonic acid polymer continuous membrane on a porous support in which the pendant ionomer moieties in said hydrated continuous membrane contain metal cations. Enhanced gas separation factors for CO.sub.2, CH.sub.4, O.sub.2, N.sub.2, and He are demonstrated with this composition. No facilitated transport mechanism is described and the use of this method and these constructions for facilitated transport are not suggested. Swelling levels, as indicated by hydration conditions, are below levels desirable for facilitated transport. No module construction is disclosed nor are composite constructions defined wherein dimensional changes on swelling are such that the membrane can be swollen after module construction.
B. K. Dutta and S. K. Sikdar have reported in Proceedings of Biochemical Engineering VI, Santa Barbara, CA (1988) on the "Separation of Amino Acids Using Composite Ion Exchange Membranes". In that study, the amino acids dissolved in the water, swelling the IEM, and diffused at different rates through the membrane. The authors described the casting of a 0.3-0.5 mil thick perfluorosulfonic acid polymer film on an EPTFE backing of undisclosed characteristics. The composites were tested as small single layer sheets, not modules. Facilitated transport was not described nor were treatments used that would secure the high levels of swelling desirable for facilitated transport.
Japanese Patent Applications 62-240627 and 63-99246 describe an EPTFE/perfluoro ion exchange polymer composite membrane construction for the drying of air and other fluid separations. Japanese Patent Application No. 63-16199, and its corresponding U.S. Pat. No. 4,875,908, describe and claim a membrane process for selectively separating water vapor from multiple component gaseous mixtures. The membrane preferably comprises a perfluoro ion exchange polymer laminated to a support layer such as porous EPTFE. Japanese Patent Application 63-62017 (1988), its European counterpart, EPO Application No. 89101201.5, and its corresponding U.S. Pat. No. 4,909,810, describe and claim a composite EPTFE/perfluoro ion exchange polymer membrane that is water vapor selective. A composite membrane is disclosed wherein the interior and exterior walls of the porous component of the composite are coated with a resin having high water content and containing ion exchange groups. None of these disclosures describes the use of carriers or the use of these composites for facilitated transport. No modules using multilayers are described nor are composite constructions defined to insure that dimensional changes on swelling are such that the membrane can be swollen after module construction and still achieve a high degree of swelling desirable for high facilitated transport flux rates.
U.S. Pat. No. 4,954,388 discloses a fabric reinforced composite membrane used as a thin continuous barrier in facilitated transport applications. The fabric reinforced composite membrane involves a three-layered structure in which the fabric is bonded to an EPTFE layer which, in turn, is bonded to a continuous perfluoro ion exchange polymer film. However, the fabric "shadows" and effectively reduces the area available for diffusion or transport. The fabric also blocks a free flow of the fluids over the entire surface of the membrane and precludes insuring a fresh supply of the feed mixture to the membrane surface or, if on the downstream side, an efficient sweep of the transported species away from the surface of the membrane.
The composites of the present invention can be swollen to high levels needed for facilitated transport corresponding to those achieved with unmodified perfluoro ionomer films and obviate many of the problems associated with prior membranes. These composites have improved mechanical strength characteristics over unmodified perfluoro ionomers with the result that thinner barriers and higher transport rates are attainable. Surprisingly, though swelling levels as high as that for unmodified perfluoro ionomers can be achieved, dimensional changes attendant to swelling do not preclude the construction of multilayer modules involving highly swollen membranes.